This competing continuation proposal is one-half of a jointly proposed two-site study by Foa and Liebowitz, who are submitting separate similar proposals. Obsessive-compulsive disorder (OCD) is chronic, debilitating, and prevalent. Controlled trials indicate that clomipramine (CMI) and exposure behavior therapy (BT), the two best established treatments for OCD, each help about 50% of treatment seekers. BT may help patients partially or unresponsive to CMI and may also reduce the high relapse rate with CMI discontinuation. To date, a conclusive comparison of CMI, BT, and their interactive effects is not available. Begun in 1990, this research compares BT, CMI, CMI + BT, and pill placebo (PBO) at a BT oriented site and at a pharmacologically oriented site, thus providing state-of-the-art treatment in each mode. This collaboration has been successful to date. Notably, however, higher than expected refusal, exclusion, and attrition rates have resulted in lower than expected enrollment. Therefore, we propose to extend the study for 3 years to accrue intent-to-treat and completer samples of sufficient size to allow adequate power to test our original hypotheses. The goals are to: 1) compare the efficacy of CMI, BT, CMI + BT, and OCD in a 12-week trial; 2) compare relapse after B and CMI are discontinued by following responders for an additional 12 weeks; 3) compare the efficacy of CMI and BT for OCD at the two centers; and 4) explore predictors of response to treatment. In the proposed research, we will study 84 patients (42 in Philadelphia and 42 in NY). When these are combined with the 85 enrolled in the previous 5 years of the project, the total N will be 169 (approximately 46 per active treatment group and approximately 29 in the PBO group). This N will allow sufficient power to test treatment and site effects. This study offers a model for pharmacotherapy-psychotherapy comparisons because experts in each modality deliver both treatment modalities. This guards against expert bias for either treatment. Its results promise to allow definitive conclusions about the relative efficacy of BT and CMI, to clarify the generalizability of both treatments and to influence clinical practice widely.